1. Field of the Invention
The present invention relates to an electric fuse, and more particularly to a MOS electric fuse which is configured to short-circuit a gate dielectric film and which is suitably applied to a MOS semiconductor device.
2. Description of the Related Art
There has conventionally been known a semiconductor memory enabling electric data writing (PROM) which uses a fuse element as a storage element. The semiconductor memory of such a type is classified into a fuse ROM which fuses the fuse element to store information, and a fuse ROM which stores information making an insulator to a conductor by dielectric breakdown (may be referred to as antifuse).
The fuse ROM that breaks down the insulator to store the information is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication. No. 7-176703. In this example, a fuse of a 3-layer structure that sandwiches a silicon layer by refractory metal layers is disposed on a semiconductor substrate, and the silicon layer is converted into a silicide compound of low resistance by supplying a large current to the fuse, and the refractory metal layers are short-circuited from each other.
Additionally, there has been known an electric fuse which electrically connects a source/drain of a MOS transistor to its substrate, applies a high voltage between the source/drain and a gate electrode to break down a gate dielectric film, and uses a resistance change therebetween. The breakdown of the fuse element to set a conductive state is called programming. For example, a gate of a PMOSFET is grounded, and a high program voltage is applied to the source, the drain and the substrate. In this case, a channel is formed in the PMOSFET. When the applied voltage becomes higher, a withstand limit of the gate dielectric film is exceeded to break down the insulator, thereby making the gate film conductive. In this way, the electric fuse is programmed.
In a large-capacity memory, a defective bit replacing technology that uses a redundancy circuit is indispensable. A fuse is used to store defective addresses. As a fuse of this type, there has been known a method which fuses a polysilicon wiring or a transistor.
The programming of the MOS transistor type fuse is classified into two cases, that is, a case in which dielectric breakdown of the gate dielectric film occurs on the source (drain) and a case in which dielectric breakdown occurs on the channel. Not only electric characteristics are different between the aforementioned two cases, but also resistance values between the terminals are different depending on a position of the broken down dielectric film. When such a variance occurs in fuse electric characteristics after the breakdown of the dielectric film, a voltage margin is reduced at the reading time of the fuse element to cause a reduction in yield or reliability.
Thus, there has been a demand to realize a MOS transistor type fuse which can make electric characteristics of an electric fuse uniform by keeping a breakdown mode thereof constant.